Syria's civil war is becoming the defining foreign policy challenge of President Obama's second term.

ROBERT SIEGEL, HOST:

The civil war in Syria is becoming the defining foreign policy challenge of President Obama's second term. Despite the mounting death toll, the president remains cautious about committing weapons and troops. NPR's Ari Shapiro has this look back at how Obama's response to Syria has unfolded.

ARI SHAPIRO, BYLINE: In early 2011, Syria was just one uprising in a region that was churning with them. In January, protesters ousted the president of Tunisia. In February, Egyptians forced their leader to step down. Libyan protesters marched in the streets. They would ultimately topple their regime with U.S. help. And in March, Syrians took to the streets of Damascus. This YouTube video showed a peaceful demonstration on March 15, 2011.

(SOUNDBITE OF VIDEO)

UNIDENTIFIED GROUP: (Chanting)

SHAPIRO: Protests grew and, at the height of the Arab Spring, President Bashar al-Assad responded with violence. So President Obama issued a stern warning. But as with many key moments in this saga, Obama first expressed his views in writing, not on camera. The April statement said: This outrageous use of violence to quell protests must come to an end now. Those words did nothing even as Obama repeated the sentiment again and again. Here's how he put it in a State Department speech about the changes sweeping the region.

PRESIDENT BARACK OBAMA: The Syrian government must stop shooting demonstrators and allow peaceful protests. They must release political prisoners and stop unjust arrests.

SHAPIRO: In the same part of that speech, Obama articulated another theme that would define his response to Syria.

OBAMA: And we have learned from our experience in Iraq just how costly and difficult it is to try to impose regime change by force, no matter how well-intentioned it may be.

SHAPIRO: That spring, the U.S. and Europe passed sanctions. Obama touted them, but they didn't work. And five months later, Obama issued another written statement: The time has come for President Assad to step aside, he said. But Assad did not step aside. As the violence grew, a presidential election was playing out here at home, and Republicans criticized Obama for moving too slowly. At a presidential debate, Romney said he would arm rebels sympathetic to the United States.

(SOUNDBITE OF ARCHIVED AUDIO)

MITT ROMNEY: Organize them, bring them together in a form of, if not government, a form of council that can take the lead in Syria and then make sure they have the arms necessary to defend themselves.

SHAPIRO: Behind closed doors, the leaders of Obama's national security team urged the same course: arm the rebels. But Obama said no, for fear of unintentionally arming Islamic extremists. At a Senate hearing, Republican John McCain put this question to Defense Secretary Leon Panetta and Joint Chiefs of Staff Chairman Martin Dempsey.

(SOUNDBITE OF ARCHIVED AUDIO)

SENATOR JOHN MCCAIN: Did you support the recommendation by Secretary of State - then Secretary of State Clinton and then head of CIA General Petraeus that we provide weapons to the resistance in Syria? Did you support that?

SECRETARY LEON PANETTA: We did.

MCCAIN: You did support that.

GENERAL MARTIN DEMPSEY: We did.

SHAPIRO: Last summer, American intelligence suggested that Assad was preparing to use chemical weapons. Several administration officials from the secretary of State down called chemical weapons a red line. And at a White House news conference in August, Obama used the phrase himself.

OBAMA: A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus. That would change my equation.

SHAPIRO: He was careful not to say how it would change his calculus. As months went by, reports of chemical weapon attacks started trickling out of Syria. They were hard to confirm. But last month, the White House told Congress that intelligence agencies believe the Syrian regime has used the chemical agent sarin. During a photo session with Jordan's king in the Oval Office a few weeks ago, Obama said there are still many unanswered questions.

OBAMA: Knowing that potentially chemical weapons have been used inside of Syria doesn't tell us when they were used, how they were used. Obtaining confirmation and strong evidence, all of those things we have to make sure that we work on with the international community.

SHAPIRO: Obama's critics replied that his red line seems to have been written in disappearing ink. At the White House this week, Obama summoned the memory of Iraq, warning that intelligence reports about weapons of mass destruction are not always accurate. Still, he said, the U.S. will follow through on its promises.

OBAMA: Whether it's bin Laden or Gadhafi, if we say we're taking a position, I would think at this point the international community has a pretty good sense that we typically follow through on our commitments.

SHAPIRO: After two years and nearly 100,000 deaths in Syria, Obama said there's an understandable desire for easy answers. But at this point, there are none. Ari Shapiro, NPR News, Washington.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.